Learning Our Place Among the Stars

I have a vague memory of a time early in my life where I thought my neighborhood had it all. The house I lived in, my friend down the block, the playground, my church, the grocery store, the pizza spot, the McDonalds, and every restaurant I had been to (I was very into food at this point of my life). I Found it difficult to imagine anything outside of this that I needed. My father worked in aerial photography, letting me see photos of Denver from the air inspiring the notion there was more out there. When I visiting a playground across town, I started to think that everything else was all more of the same, Denver maybe had a few neighborhoods each one with a playground, I of course wanted to see them all to make sure I had the best.

I had a globe but never saw it as more than a spinning toy till I was shown on it where my grandmother lived (Texas). I then got somewhat of an idea of what all was out there, how many playgrounds I have yet to see. I’ve traveled a bit of the world now but I’m still reminded of how tiny that bit is all the time. Even at home, I have lived in Colorado for the vast majority of my life and still find surprises in my own backyard. Collectively humanity has been discovering the universe in a much similar way.

In biblical times the sky was seen as a solid dome and the Earth a flat plane below it. Genesis 1:6-8 (written between the 6th and 5th centuries BC) describes the creation of the sky: “Then God commanded, ‘Let there be a dome to divide the water and to keep it in two separate places’—and it was done. So God made a dome, and it separated the water under it from the water above it. He named the dome ‘Sky.’ ”.

The word for sky was firmament which you can break down and see the word ‘firm’ showing the idea of the sky as a solid. This is a Latin word translated from the Greek stereoma meaning “solid dome” which is a translation from the Hebrew rakia meaning “thin metallic sheet”

The Flammarion engraving (1888)

In the later Revelations 6:13-14 (written ~90 CE) the sky is described as rolling up and the stars falling off it “The stars fell down to the Earth.... The sky disappeared like a scroll being rolled up…”

In the 5th century BC Greek philosophers were starting to pick up on the Earth being round and imagined the sky as a round sphere that spun once every 24hrs around the round. The stars were seen as fixed to the sphere and the planets, Sun, and moon wandered around the sphere.

There were a few Greek scholars that around the 3rd century BC that suggested other ideas. Democritus of Adbera who in 490BC speculated The milky way was made up of many small stars. The common thought of the time was that the milky way was a luminous fog on the sphere of the sky. Even Aristarchus of Samos in 260BC was the first known to suggest the planets revolve around the Sun, not Earth. However these speculations didn’t gain traction in their time as most were happy with the sky being a giant sphere revolving around the Earth with everything else in the sky. It wasn’t till very recently (in the context of time) that we started to shape how we see the sky to this day. Astronomer Nicolaus Cusanus (in 1440), and italian Giordano Bruno (in the late 1500s) introduced us to the notion of space being a place stretching infinitely, and all the stars are much like our Sun so far away we just see them as specks of light. Introducing this idea was not easy as the solid sphere model had been heavily ingrained in the minds of the masses.

Statue of Giordano Bruno in the location of where he was burred at the stake for his beliefs which are accepted as fact today. The sculptor controversially made him glaring at the Vatican.

In 1543 astronomer Nicolaus Copernicus pointed out that placing the Sun in the center of the planets made it easier to predict the future positions of the planets. However, this was seen more as a mathematical trick and didn’t persuade people to really viewing the Sun at the center. Many who thought the Sun in the center of the planets still thought of the stars as fixed to a sphere. Once Galileo Galilei started exploring the heavens in 1609 with a telescope that evidence for both of these speculations started to pile up. Looking at the milky way he saw that it truly was made up of countless faint of stars. Looking at Jupiter he saw that it had 4 moons that orbit it independent of the orbit around the Earth, and seeing Venus has phases just like our Moon was much easier described by having the Sun in the center rather than the Earth.

Illustration of the Copernican system(1708). The sun is in the center but the outer stars are still on an outer sphere.

Also in 1609 astronomer Johann Kepler refined the model of the Sun centered (heliocentric) solar system by showing mathematically that the planets orbit the Sun in ellipses, not perfect circles. This means we drift closer and further to/from the Sun, and planets at different points in our orbit. This can be seen in “super Moons” which appears larger than the regular full Moon because the Moon happens to be on the point on its elliptical orbit that makes it closer to Earth.

Astronomer Giovanni Domenico Cassini in 1672 made the first measurements to the size of the solar system. He estimated Saturn (the farthest known planet at the time) to be around 3 billion kilometers away. This was way larger than anyone at the time would have guessed.

None of this changed the concept of what was beyond the planets. It wasn’t until 1718 when astronomer Edmund Halley was making a new star map and found some stars (Prycon, and Sirius from a blog post a few weeks ago) had moved a little. Ancient Greek star charts had them placed slightly differently among the dimmer stars than what Halley was observing. This showed that the stars were moving too!

Halley picked up the speculation that the stars were just like our Sun but far away. Assuming Sirius is the same brightness as our Sun Halley calculated it would have to be 2 light years away (making him the first to measure distances in light years). Sirius is much brighter than our Sun so this was way off, Sirius is more like 8.6ly away.

In 1838 to get a real Idea of the distance to the stars astronomer Friedrich Wilhelm Bessel used paralax. Just like your two eyes gives you depth perception, parallax gives astronomers depth perception to the stars. If you look up at a star close one eye and cover it with your thumb then, switch eyes you will see your thumb move and no longer cover the star. Your brain takes these two images and realizes your thumb is closer than the star because it moves. To do this with the stars we need two images from much further apart, and our orbit around the Sun gives us that. If we take a picture of the sky tonight then, wait 6 months when the Earth is on the other side of the Sun, halfway around our orbit, and take another picture we’ll have two images of the sky from each side of the Sun. Now comparing these two images we might notice some stars move very slightly while others appear to not move at all. Knowing the Earth-Sun distance and the angle at which the closest stars move we can get a highly accurate measurement of the distances to the stars.

an example of parallax

Bessel found these distances to be astonishingly huge! The closest Alpha Centauri was 4.3 light years away. In doing so Bessel finally smashed the idea of a sphere with stars outside the planets, and tremendously increased the size of the Universe.

Measurements of the galaxy started to take shape looking at the milky way which makes a circle around the sky it was apparent that stars preferred this region of the sky. Scholar Immanuel Kant in 1755 proposed that just like the planets orbit the Sun in a flat ring maybe the stars orbit the milky way in a flat plane. He also concluded because the milky way extends all the way around Earth we must be towards the center of the galaxy. In 1785 astronomer William Herschel confirmed Kant’s speculation by taking a survey of the stars. He called this star system we are in the “galaxy”. Nebulae were well known and studied objects at the time, while categorizing nebula there were some known as ‘spiral nebulae’ that didn’t quite fit. Some of these nebula such as the Magnetic clouds and “The great nebula in Andromeda” could be seen with the naked eye. Kant proposed that these were other star systems or “Island Universes”. This conjecture was debated among astronomers until 1936 when astronomer Edwin Hubble, using the new 100-inch telescope, was able to estimate the distance of these nebulae and found them to be much further than any thing that could be within our own galaxy.

Like my realization that I will never get to visit every playground, in 1936 was when accepted everything we know, our galaxy, was just a tiny fraction of what was out there. Our Earth is no longer in the center of the planets, our Sun is no longer the center of our galaxy and our galaxy is no longer unique. As of 2016 NASA estimates there to be 2 trillion galaxies in the observable universe.

Diving Deep Inside Uranus

Let's get the jokes out of the way: In this blog post will be diving deep inside Uranus, Uranus has a dark spot, Uranus has a ring around it, and of course Uranus is a gas giant surrounded by a cloud of methane. Now to ruin this fun most people say yoo-rain-us or more crassly ur-anus when talking about the 7th planet from the sun. I usually say oo-ron-us which is closer to the Greek pronunciation and side steps the giggles. Say it how ever you feel, I just want this article to entertain you and jokes are entertaining.

Uranus’s name comes from Greek mythology just like all of the planets (the Romans renamed them to their gods who were just borrowed from Greek mythology). The Greek names for the planets are: Mercury = Hermes, Venus = Aphrodite, Earth = Gaia, Mars = Ares, Jupiter = Zeus, Saturn = Cronus, Uranus = Uranus, Neptune = Poseidon, Pluto = Hades. There’s a subtle family tree here Hermes, Aphrodite, and Ares are all children of Zeus, Zeus is Cronus’s son, so it would follow that the next planet out would be named after Cronus’s father. Uranus is the god of the sky. The myth is Uranus (the god of the sky) and Gaia (the god of the earth, “mother earth”) are the first primordial gods which everything came from.

Uranus the god of eternity, is standing inside a celestial sphere decorated with zodiac signs, in between a green tree and a bare tree (summer and winter, respectively). Sitting in front of him is the mother-earth goddess, Tellus (the Roman counterpart of Gaia)

Uranus was the first planet to be discovered in a telescope, and with that the first planet to be discovered at all. All of the planets closer to the sun than Uranus are so obvious in the night sky that any one that spends a bit of time outdoors (as our ancestors did) would notice them wandering in the night sky. However Uranus is so far and small you can just barely see it without a telescope.

Uranus was discovered by William Herschel in 1789. William Herschel was a brilliant self-educated astronomer who built his own telescopes because he couldn’t afford one. He ended up so good at this that he built the best telescope around in his time. He made many discoveries including two moons of Saturn, and Mars axis was tilted just like Earth’s.

Friedrich Wilhelm Herschel 1738 - 1822

While studying the stars in Gemini Herschel noticed one that appeared as a disk in his telescope. You see stars are so far away that even the best telescopes can’t see them as more than a point of light. Planets are close enough we see some surface detail. In perspective Uranus would be like looking at a sesame seed from 200 yards away (2 football fields), which can be done with a powerful telescope. Where looking at the closest star (Alpha Cen) would be like looking at a grapefruit 2,000 miles away (about the distance from boulder to Belize) which even the best telescopes can’t do. We see stars as points of light because they are so far but really bright and we see planets as disks because they are close enough to resolve their surface. Herschel was very hesitant to say he discovered a planet. He tried to call it a commit but, it was a well defined circle, not a haze that you would expect from a commit. Even after he had enough observations to see it had a circular orbit (not the round orbit expected of a comet) he hesitated in saying it was a planet. It was actually another astronomer (Anders Jean Lexell) that claimed Herschel had discovered a planet beyond Saturn. Herschel was given the right to name the new planet as the discoverer but he wanted to name it after the British king George III, and called it “Georgium Sidus” of “George's Star”. He obviously knew who would write him the biggest check. I also want to point out he never wanted to call Uranus just “George” I see this in Trivia from time to time, and not in any primary sources. While a planet named George is a funny prospect it would've been disrespectful to call a monarch by their first name. Astronomers outside of the UK disliked naming it after a British monarch and other names were proposed. The most notable of the other names is calling it Herschel after the discoverer. However the tradition of naming planets after Greek gods was given favor and the name Uranus outlasted all of these names. Uranus wasn’t discovered for so long for a few reasons, all mainly having to do with it’s distance from the sun. Uranus is twice as far from the sun as Saturn (it’s discovery doubled the size of our solar system) and half the size of Saturn. This makes Uranus .36% the brightness of Saturn. Saturn is the ninth brightest thing in our sky there’s a few stars (our sun, Sirius, and Canopus), and of course the moon and other planets that are brighter than Saturn. However there’s about 5,000 stars that are brighter than Uranus, it’s easily looked over.

Another thing that sets planets apart is they move among the background stars which stay still (the word planet actually means “wanderer” or wandering star). Uranus is no exception however it is much slower around the sun than the other planets. It takes Uranus 84 years to orbit the sun (meaning for every 84 times earth goes around the sun Uranus goes once), where Saturn takes about 30 years to orbit the sun, Jupiter about 12 years, and mars about 2 years. This means it takes Uranus 44.4 days to move the width of one full moon in our night sky, where Saturn takes 15 days, Jupiter 6.25 days and mars about a day.

Table of apparent motion of Uranus compared to the other outer planets.

Uranus is much slower to move around the zodiac (the zodiac being the only constellations that are visited by planets because, they fall on the “ecliptic”, the line that denotes the plane of our solar system). There are 12 constellations in the zodiac and Uranus takes 84 years to go thru all of them meaning Uranus spends 84/12=7 years in each sign. Because of the infrequency when Uranus transits between constellations Astrologers put more weight into it when this happens. I of course know very little of astrology however for this post I’ve teamed up with my good friend Sarah at Lives Her Vision and she has a blog post here covering the astrological side of Uranus.

The other planets motion is much more noticeable when observing month to month/year to `year than Uranus. Many astronomers saw Uranus before it was discovered and even cataloged it. In the mid 1700's an astronomer (Pierre Charles Lemonnier) recorded Uranus 12 times each time thinking it was a different star.

Uranus has many things that set it apart from the other planets. It is the only planet that rolls on its equator in its orbit rather than spins like a top. To put it a more scientific way Uranus has an axial tilt of 97.77°. If you could stand on the surface of Uranus and see the sky clearly you would see the sun spiral around the sky tracing out almost a helix pattern in the sky, this would happen very slowly and take 84 years to watch unfold completely.

Being so far from the sun means the sun wouldn’t appear nearly as bright in the sky as it does on earth. The sun would be about a 20th the size it appears in our sky and the sun would be 1/368th as bright (still 1,250 times as bright as a full Moon). Because of this Uranus is very cold, it is the coldest planet in our solar system ( Uranus is even colder than the further planet Neptune, for reasons we are not 100% sure of but likely to do with it being tilted on its side).

Uranus is radius is about 4 times that of earth's. It is mostly comprised of hydrogen and helium with frozen water, ammonia and methane present as well. Storms have been observed in the atmosphere of Uranus, the “dark spot on Uranus” I mentioned in the start of this is one such storm just like Jupiter's great red spot. Uranus is very similar in composition to its neighbor Neptune which has given the two the classification of “Ice Giants” to separate them out from Jupiter, and Saturn (the gas giants). Deep beneath Uranus’s thick atmosphere is a core that is thought to be about the size of Earth.

Size comparison

Uranus has 27 known moons all of which are names after characters from the works of Shakespeare, breaking the tradition of giving Greek names to the planetary bodies. The largest moon is Titania, which is about half the size of our moon, it’s the 8th largest moon in the solar system.

Uranus and its six largest moons compared at their proper relative sizes and relative positions. From left to right: Puck, Miranda, Ariel, Umbriel, Titania, and Oberon

Uranus has a ring system as well, it was the first in 1977 when Uranus passed in front of a star. Many astronomers were observing this event and to everyone’s surprise before Uranus covered the star, the star dimmed 5 separate times and after Uranus passed in front of the star 5 more dimming events took place. This could only mean Uranus had at least 5 rings. We’ve only gotten an up close look at Uranus from the Voyager 2 flyby in 1986, in this flyby we learned most of what is known today about the Uranian system. Voyager 2 got to see the rings up close and counted 13 of them.

Rings of Uranus seen by Voyager 2

Observing Uranus is difficult for an amateur astronomer, and when found it offers very little to be seen. A telescope of about 6 inches or larger can resolve a small pale blue disk under a magnification of 85x or greater. But that’s about all there is to see, just that it’s not a point of light like a star. If you are really patient and track it down in a telescope and then proceed to wait a few months/years and do so again you will see how slowly it moves in the night sky. If you want to see Uranus the best way would be to join me on one of my tours. I’d love to show you Uranus in my large telescopes!

You Can-opus be Sirius!?

Looking south in at night it would be hard to miss the brightest star in our night sky Sirius. Now, I say in our “night” sky because of course the sun is a star and by a large margin the brightest one in our sky. So, let’s give the sun credit: sun, you are the brightest star that lights up our life, wouldn’t be here without you.

The name Sirius comes from Greek meaning scorching as it has been the brightest star in the night sky for all of written history. Sirius will continue to be the brightest star for some 200,000 more years when the distance between between the solar system and Sirius slowly increases and Sirius fades. Sirius is also called the “dog star” as it is located in the constellation Canis Major (the big dog) who is Orion’s trusty companion following right behind him across the night sky.

They say Orion has taken a liking to the seven sisters you can watch him chase them across the night sky. However Zeus (or Jupiter if you ask the Romans) put a bull (Taurus) in between them to protect the girls. Orion is being followed as well by his two best friends his little and big dog.

On occasion you might hear hot days referred to as “dog days” this actually comes from Sirius the “dog” star and dates back to ancient Egypt. When Sirius was seen rising in the morning with the sun it was the hottest days and coincided with the flooding of the Nile. because of this Egyptians would watch for it in the mornings to know to prepare for the coming floods. It was thought that the sun and Sirius shining together is what made the heat so it was natural to call it a “dog day”.

This last week I got to visit my grandfather in southern Arizona and while doing so I had the chance to see an old friend/favorite of the night sky Canopus. Canopus is the second brightest star in the night sky, and located in the constellation Carina (the giant ship). Canopus is not visible in much of the united states. Only in the southern most states can Canopus be viewed for a brief amount of time, a few nights a year. This is because Canopus is located at a declination of 52° S meaning that anyone above the latitude 38°N (90°N-52°S=38°N) will be unable to see Canopus in their night sky. Living in Boulder which is 40°N (Baseline Rd is actually called such because it follows the 40th parallel) means I never get to see Canopus unless I travel to see it.

Table 1 – The top 10 brightest stars. The brightness has been adjusted from normal units to be with respect to Vega, so it is easier to compare. 4 being 4 times as bright as Vega (or 400% the brightness if you rather) and .64 being just under 2/3rds as bright (or 64% as bright) as Vega. I choose Vega not only because it’s my favorite but also between Arturus, Vega and Capella, which are all about the same a comparative example should be in the sky year-round. Capella is currently right over head in the evening sky making it a good comparison right now. Also of mention: stars can vary in brightness these are based on commonly accepted averages.

Demonstrated quite well on this table is the difference between absolute and apparent magnitude. If you look at Sirius and Canopus, Sirius is twice as bright as Canopus sure but, Canopus is 310 light years away where Sirius is only 8.6 ly away. Comparing Canopus to Alpha Centauri you will see Alpha Cen is slightly dimmer but at 4.4 ly even closer than Sirius. This all tells us Canopus must be bright! While I was living in Australia Canopus was common in the sky. My mentor Greg (#SpaceGandalf) would say “Respect” when ever talking about Canopus. To be the second brightest star in the night sky and 310 ly away beating a star that is 70 times closer it must be waaaaay brighter! RESPECT indeed.

This is the difference between absolute and apparent magnitude. Absolute is how bright the star would be if we compared stars from the same distance and apparent is how it appears at the distance we see it in our sky. Sirius is our brightest neighbor it would be like the street light at the end of our block. Canopus is the brightest thing in our part of our galaxy it would be like a stadium 36 blocks away.

The difference between absolute and apparent magnitude.

To put this yet another way; if you travel even 50ly from the sun you wouldn’t even be able to see it without a telescope. Our star (the sun) is actually really dim when compared to these stars, even if you traveled to Alpha Cen (4.4 ly) it wouldn't even be on the list at a brightness of 61% that of Vega.

If you want to see a pair of stars like this in our Colorado night sky take a look at Procyon and Rigel. Procyon is the brightest star in Canis Minor (the little dog) which is just above Sirius and Canis Major. Rigel is Orion’s bottom left foot. Both Rigel and Procyon are about the same brightness however, Procyon is 11 light years away and Rigel is 860 light years away.

Procyon Vs Rigel, both these stars look the same brightness however one is 849 light years further than the other

If you want to get an up close look at Sirius and learn more about our night sky consider booking an AstroTour